Water mass analysis and contribution estimation using heavy rare earth elements: Significance of Kuroshio intermediate water to Central East China Sea shelf water

The East China Sea (ECS) has complicated water masses, in which it is difficult to clarify the various sources solely by estimating the potential temperature (PT) and salinity. High-nutrient bottom water was identified on the central ECS shelf. The pattern of heavy rare earth elements (HREEs) suggested the important contribution of Kuroshio intermediate water (KIW) in this bottom water. HREEs combined with hydrographic and chemical parameters indicated that the bottom water is a mixture of KIW, Kuroshio tropical water (KTW), and mixed shelf water (MSW). The mixing ratios of these three water sources estimated by four HREEs are comparable with the results calculated by PT and salinity. Then, PT and salinity are combined with HREEs to quantify the mixing of four water masses (KTW, MSW, KIW, and Kuroshio surface water) in the entire water column on the central shelf of the ECS. Our results indicate that the high amounts of nutrients at 130 m (bottom) on the ECS middle shelf mainly originate from the 400-600-m depth of KIW and the 150-300-m depth of KTW. The fractional nutrient concentration from the KIW in the bottom water yielded NO3 + NO2: 63 ± 23% and P: 62 ± 22%, suggesting the significance of the KIW's material transport to the central ECS shelf. This study demonstrates that HREEs, in combination with conventional temperature and salinity, effectively increase the number of available tracers so that water mass analysis can be done with more robust results achieved.